Changing image size in Markdown

You can use jQuery's attr() function. For example, if your img tag has an id attribute of 'my_image', you would do this:

<img id="my_image" src="first.jpg"/>

Then you can change the src of your image with jQuery like this:

$("#my_image").attr("src","second.jpg");

To attach this to a click event, you could write:

$('#my_image').on({
    'click': function(){
        $('#my_image').attr('src','second.jpg');
    }
});

To rotate the image, you could do this:

$('img').on({
    'click': function() {
         var src = ($(this).attr('src') === 'img1_on.jpg')
            ? 'img2_on.jpg'
            : 'img1_on.jpg';
         $(this).attr('src', src);
    }
});

Do not apply an explicit width or height to the image tag. Instead, give it:

max-width:100%;
max-height:100%;

Also, height: auto; if you want to specify a width only.

Example: http://jsfiddle.net/xwrvxser/1/

img {
    max-width: 100%;
    max-height: 100%;
}

.portrait {
    height: 80px;
    width: 30px;
}

.landscape {
    height: 30px;
    width: 80px;
}

.square {
    height: 75px;
    width: 75px;
}

Portrait Div
<div class="portrait">
    <img src="http://i.stack.imgur.com/xkF9Q.jpg">
</div>

Landscape Div
<div class="landscape">
    <img src="http://i.stack.imgur.com/xkF9Q.jpg">
</div>

Square Div
<div class="square">
    <img src="http://i.stack.imgur.com/xkF9Q.jpg">
</div>